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Electrophysiology

Electrophysiology. Electroencephalography. Electrical potential is usually measured at many sites on the head surface More is sometimes better. Magnetoencephalography. MEG systems use many sensors to accomplish source analysis

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Electrophysiology

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  1. Electrophysiology

  2. Electroencephalography • Electrical potential is usually measured at many sites on the head surface • More is sometimes better

  3. Magnetoencephalography • MEG systems use many sensors to accomplish source analysis • MEG and EEG are complementary because they are sensitive to orthogonal current flows • MEG is very expensive

  4. EEG/MEG • EEG changes with various states and in response to stimuli

  5. EEG/MEG • Any complex waveform can be decomposed into component frequencies • E.g. • White light decomposes into the visible spectrum • Musical chords decompose into individual notes

  6. EEG/MEG • EEG is characterized by various patterns of oscillations • These oscillations superpose in the raw data 4 Hz 4 Hz + 8 Hz + 15 Hz + 21 Hz = 8 Hz 15 Hz 21 Hz

  7. How can we visualize these oscillations? • The amount of energy at any frequency is expressed as % power change relative to pre-stimulus baseline • Power can change over time 48 Hz % change From Pre-stimulus 24 Hz 16 Hz Frequency 8 Hz 4 Hz +200 +400 +600 0 (onset) Time

  8. Where in the brain are these oscillations coming from? • We can select and collapse any time/frequency window and plot relative power across all sensors Win Lose

  9. Where in the brain are these oscillations coming from? • Can we do better than 2D plots on a flattened head? • we (often) want to know what cortical structures might have generated the signal of interest • One approach to finding those signal sources is Beamformer

  10. Beamforming • Beamforming is a signal processing technique used in a variety of applications: • Sonar • Radar • Radio telescopes • Cellular transmision

  11. Beamformer • Applying the Beamformer approach yields EEG or MEG data with fMRI-like imaging R L

  12. The Event-Related Potential (ERP) • Embedded in the EEG signal is the small electrical response due to specific events such as stimulus or task onsets, motor actions, etc.

  13. The Event-Related Potential (ERP) • Embedded in the EEG signal is the small electrical response due to specific events such as stimulus or task onsets, motor actions, etc. • Averaging all such events together isolates this event-related potential

  14. The Event-Related Potential (ERP) • We have an ERP waveform for every electrode

  15. The Event-Related Potential (ERP) • We have an ERP waveform for every electrode • Sometimes that isn’t very useful

  16. The Event-Related Potential (ERP) • We have an ERP waveform for every electrode • Sometimes that isn’t very useful • Sometimes we want to know the overall pattern of potentials across the head surface • isopotential map

  17. The Event-Related Potential (ERP) • We have an ERP waveform for every electrode • Sometimes that isn’t very useful • Sometimes we want to know the overall pattern of potentials across the head surface • isopotential map Sometimes that isn’t very useful - we want to know the generator source in 3D

  18. Brain Electrical Source Analysis • Given this pattern on the scalp, can you guess where the current generator was?

  19. Brain Electrical Source Analysis • Given this pattern on the scalp, can you guess where the current generator was? Duracell

  20. Brain Electrical Source Analysis • Source Analysis models neural activity as one or more equivalent current dipoles inside a head-shaped volume with some set of electrical characteristics

  21. Brain Electrical Source Analysis Project “Forward Solution” This is most likely location of dipole Compare to actual data

  22. Brain Electrical Source Analysis • EEG data can now be coregistered with high-resolution MRI image

  23. Intracranial and “single” Unit • Single or multiple electrodes are inserted into the brain • “chronic” implant may be left in place for long periods

  24. Intracranial and “single” Unit • Single electrodes may pick up action potentials from a single cell • An electrode may pick up thecombined activity from several nearby cells • spike-sorting attempts to isolate individual cells

  25. Intracranial and “single” Unit • Simultaneous recording from many electrodes allows recording of multiple cells

  26. Intracranial and “single” Unit • Output of unit recordings is often depicted as a “spike train” and measured in spikes/second Stimulus on Spikes

  27. Intracranial and “single” Unit • Output of unit recordings is often depicted as a “spike train” and measured in spikes/second • Spike rate is almost never zero, even without sensory input • in visual cortex this gives rise to “cortical grey” Stimulus on Spikes

  28. Intracranial and “single” Unit • By carefully associating changes in spike rate with sensory stimuli or cognitive task, one can map the functional circuitry of one or more brain regions • What are the advantages and limitations of this approach?

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